Differential signal transmission, in general, is a known method of transmitting information electrically from a transmitting device (or transmitter) to a receiving device (or receiver), by means of two complementary signals sent on two separate traces in which the receiver recovers the information based on the difference between the two complementary signals. In general, the receiver ignores the traces' voltages with respect to ground, therefore, small changes in ground potential between transmitter and receiver do not affect the receiver's ability to detect the signal. Therefore, better tolerance of ground offsets, low switching noise and better noise immunity under low voltage levels may be achieved with differential transmission. The technique may be used in both analog signaling, as in some audio systems, and digital signaling, as in high-speed serial I/O interfaces.
A pre-driver and a driver generally refer to electronic circuits of a transmitter used to condition and transmit the conditioned differential signals to a receiver. A predriver may predrive a signal to improve the characteristics and/or quality of the signal before it is delivered to a driver for transmission to the receiver.
In high speed transmission, there are losses in the transmission lanes. In order to compensate for the losses, driver circuits associated with the transmission lanes may use “deemphasis” or “pre-emphasis” to reduce the transmit data amplitude for repeating bits of the transmit data. More specifically, transmit data may be driven at full amplitude (full-swing mode) for each polarity (or state) transition (e.g., 0 to 1, 1 to 0, 1 to −1, −1 to 1, etc.) and thereafter repeating bits of the same polarity (or state) may be driven at reduced amplitude (deemphasis mode). For example, PCI Express Base Specification, Revision 2.1, published Mar. 4, 2009, mandates de-emphasis treatment of repeating bits of same polarity. For another example, VESA DisplayPort Standard, Version 1, Revision la, released Jan. 11, 2008, requires pre-emphasis treatment of repeating bits.
In general, two relatively important requirements of a predriver for high-speed differential interfaces may be 1) a matched up/down skew with the main differential driver output signals, and 2) a stable common-mode voltage. An issue associated with high-speed differential transmitters may be that the speed of a down output driver transition may not match the speed of an up output driver transition. For example, for a NMOS type driver where the current source is connected to the ground, a down output driver transition may be faster than an up output driver transition. This may occur due to the down output driver transition being dominated by strong current sources quickly discharging highly capacitive output nodes whereas the up transition being dominated by passive resistors, commonly implemented via on-die termination (ODT), charging the opposite output nodes at a slower pace. Similarly, for a PMOS type driver, where the current source is connected to the Vcc, an up output driver transition may be faster than a down output driver transition. In both cases, this may cause up/down output signal slope mismatch between the predriver and the driver, which may lead to higher jitter, less system margins and degradation of overall performance.